(a) Field of the Invention
The present invention relates to an image display apparatus having pixels of which the brightness can be controlled by a signal, that is, an image display apparatus with pixels each having a light-emitting element such as an organic EL (Electro-Luminescence) element of which the brightness can be controlled by a current. More specifically, the present invention relates to an active matrix type image display apparatus that controls the amount of current supplied to the light-emitting element using an active element such as an insulated gate type field effect transistor provided in each pixel.
(b) Description of the Related Art
In general, an active matrix type image display apparatus has a plurality of pixels in matrix form and controls intensity of light for each pixel according to given brightness information so as to display an image. As for an image display apparatus using liquid crystals as an electro-optic material, the transmittance of each pixel is variable depending on the voltage recorded in the pixel. The active matrix type image display apparatus using an organic EL material as an electro-optic material has the same basic operation as the liquid crystal display devices. Unlike the liquid crystal display devices, however, the organic EL image display apparatus is a self-luminous type that has a light-emitting element such as an OLED (Organic Light-Emitting Diode) in each pixel and exhibits high visibility of images and high response speed without a need for backlights. The brightness of each light-emitting element is controlled by the amount of current. For example, the organic EL image display apparatus has a striking difference from the liquid crystal display devices in that the light-emitting element is of a current-driven or current-controlled type.
Like the liquid crystal display devices, the organic EL image display apparatus uses either a simple matrix type driving method or an active matrix type driving method. The simple matrix type driving method is simple in structure but has a difficulty in realizing a large-size display device and high resolution which has led to the recent demand for the earnest development of active matrix methods. In the active matrix type driving method, the current flowing to the light-emitting element in each pixel is controlled by an active element (usually a TFT (Thin Film Transistor), which is a kind of insulated gate field effect transistor) provided in the pixel.
A variety of pixel structures have been suggested in approaches to compensate for the inter-pixel characteristic deviation of the threshold voltage of the TFT used as an active element for controlling the current flowing to the light-emitting element. The pixel structure using a current mode program system is one of them.
FIG. 1 shows a pixel structure applied to the current mode program type image display apparatus according to prior art. The pixel structure of FIG. 1 is an equivalent circuit for one pixel.
As illustrated in FIG. 1, the pixel is formed at the intersection of scan and data lines. A signal Scan for selecting the pixel is applied to the scan line with a predetermined scanning cycle, and brightness information for driving the pixel is applied in the form of a current Idata to the data line. The pixel includes one OLED used as a light-emitting element, four TFTs M1 to M4, and one storage capacitor Cst.
Once the scan line on which the pixel is positioned is selected according to the signal Scan, both the transistors M2 and M3 are turned on and the transistor M4 for controlling whether to supply the current to the OLED is turned off. The current Idata including brightness information and supplied through the data line is provided to the pixel via the transistor M3 in the “on” state. The difference between this current and a current flowing to the transistor M1 is fed back to the gate electrode of the transistor M1 via the transistor M2 in the “on” state. Then, a voltage corresponding to the current Idata is recorded on the storage capacitor Cst coupled between the gate and source electrodes of the transistor M1.
Once the scan line is unselected, the transistors M2 and M3 are turned off and the transistor M4 is turned on. The turn-off switching of the transistor M2 makes the gate electrode of the transistor M1 float and sustains the voltage recorded on the storage capacitor Cst. The transistor M1 operates in saturation region to generate a drain current according to a gate voltage. The current generated by the transistor M1 flows to the OLED via the transistor M4 in the “on” state, and the degree of light emission of the OLED is determined by the amount of the current, thereby representing a desired brightness.
In the above-described current mode program type image display apparatus according to prior art, the current for driving the data line must be equal to the current flowing to the OLED, taking a long time to drive the data line. In other words, the current mode program type image display apparatus may compensate for the characteristic deviation of mobility as well as that of threshold voltage of the transistors used in the pixel, but it takes too much time to drive the data line at a low current level and has a limitation in realizing a high-gradation and high-resolution image display apparatus.
FIG. 2 shows an image display apparatus having a pixel structure using an asymmetric current mirror for solving the above-mentioned problems.
The pixel of FIG. 2 is formed at an intersection of scan and data lines. Two scan lines are arranged for a pixel of one row. Signals Scan1 and Scan2 for selecting the pixel are applied to the scan lines with a predetermined scan cycle, and brightness information for driving the pixel is applied in the form of a current Idata to the data line. The pixel includes an OLED used as a light-emitting element, two TFTs M1 and M2 that form a current mirror, a storage capacitor Cst for storing brightness information converted from the current Idata at a voltage level, and transistors M3 and M4 for controlling the supply of the current Idata to the transistor M2 and the storage capacitor Cst, respectively.
For selecting the pixel, the signals Scan1 and Scan2 transferred via the two scan lines have a cycle for turning on the two transistors M3 and M4 almost simultaneously. The current Idata including bright information that is applied to the data line by the turn-on switching of the transistor M3 flows to the transistor M2. The turn-on switching of the transistor M4 causes a short circuit between the gate and drain electrodes of the transistor M2. The transistor M2 operates in saturation region, and a gate-source voltage corresponding to the current Idata is generated by a feedback via the transistor M4 and recorded on the storage capacitor Cst. When the two scan lines are unselected, the two transistors M3 and M4 are turned off to make the gate electrode of the transistor M2 float and sustain the voltage recorded in the storage capacitor Cst. The voltage sustained at the storage capacitor Cst is applied to the gate of the transistor M1 to generate a drain current, by which the OLED is driven.
In the image display apparatus having the above-stated pixel structure, the channel width of the transistor M2 that forms the current mirror is greater than that of the transistor M1 driving the OLED, or the channel length of the transistor M1 is greater than that of the transistor M2. In this manner, the current flowing to the transistor M2 is higher than that flowing to the transistor M1 in a predetermined proportion. Hence, the OLED can be driven with a current having a magnitude in a desired brightness range, while increasing the current used for driving the data line. But the current flowing to the data line must be several tens of times higher than the current flowing to the OLED because of a high load caused by the parasitic capacitance and the parasitic resistance of the data line. With a high ratio between the current flowing to the data line and the current flowing to the OLED, the required time for driving the data line is shortened but the size of the transistor that forms the current mirror is increased. Hence, there is a problem in that it is difficult to acquire a high aperture ratio, for example, when using a bottom emission system.